1. Field of the Invention
The present invention relates to a color processing apparatus and method for performing color conversion according to a viewing condition.
2. Description of the Related Art
In recent years, devices which handle color images have become more widely used. In conjunction with this, a common workflow is to perform the design and layout of images digitalized by a digital camera or a scanner on a monitor for output using a printer. This workflow is widely carried out in fields such as desktop publishing (DTP), computer graphics (CG) and computer-aided design (CAD). During such processing, the range of colors which can be reproduced (hereinafter, referred to as “color gamut”) differs for each device handling an image. Therefore, there is a need to perform color conversion processing for making up the difference in color gamuts among devices in order to match the colors of the image in each step.
A color matching system (CMS) is employed for the color conversion processing which matches colors among different devices. As illustrated in FIG. 18, a CMS is formed from a device profile that lists the characteristics and viewing conditions of the devices to be matched and a color matching module (CMM) for performing the matching processing of the colors among those devices. The CMS, first, converts input device-dependent color signal values (RGB, CMYK etc.) to device-independent color signal values (CIELAB, CIECAM02 etc.) During the conversion, the viewing condition (appearance parameter) listed in the device profile is used. Examples of parameters for conversion into CIELAB color space include white tristimulus values, and examples of parameters for conversion into CIECAM02 color space include white tristimulus values, luminance of the adapting field, background luminance and surroundings. Next, color matching is carried out by performing color conversion processing (gamut mapping) to make up the difference in color gamuts among different devices in this device-independent color space.
Generally, to achieve high-accuracy color matching, the characteristics of the devices to be matched have to be measured by a chromometer so that a device profile can be generated which stores the resulting device characteristic data. However, chromometers are expensive, and it takes time to carry out the measurement. Thus, an sRGB profile which is standardized for typical monitor devices and a profile provided by the respective vendors are used.
A typical monitor profile, such as an sRGB profile for instance, assumes that the monitor is under dark conditions. Further, even when measuring a device characteristic of a monitor with a usual chromometer, the measurement is carried out by attaching the chromometer close to the screen to block out external light. Therefore, no consideration is given to the effect of external light reflection on the monitor screen which occurs under a normal monitor viewing condition, for the device characteristic listed in the device profile of such a monitor. FIG. 19 illustrates an example of external light 1901 reflection on a monitor screen 1902. Here, letting the tristimulus values of the emitted light component 1903 of the monitor 1902 be XYZm, and the tristimulus values of the reflected component 1904 on the monitor 1902 be XYZr, the tristimulus values XYZ 1905 which actually enter an observer's eye 1906 can be found according to the following equation.XYZ=XYZm+XYZr 
Specifically, a person observing the monitor perceives a color different to XYZm, which is the characteristic intrinsic to the device. Since the tristimulus values of this reflected component are constant irrespective of the emitted light component, the larger the difference perceived by an observer is, the smaller the XYZm values for a color are. FIG. 20 illustrates the shape of a monitor color gamut when no consideration is given to the reflected component on the monitor screen. For conventional color matching such as that illustrated in FIG. 20, the difference is especially large in dark sections of the image, and matching accuracy deteriorates.
Accordingly, for conventional monitor and printer color matching, no consideration is given to the effect of external light reflection which occurs according to the viewing condition with a monitor used in color matching. Therefore, there has been the problem that matching accuracy deteriorates.
Further, a technique for correcting the characteristic of a device displaying an image according to the viewing condition has been proposed (U.S. Pat. No. 6,847,374).
Although the method discussed in U.S. Pat. No. 6,847,374 improves the appearance of an image displayed on a display device, it lacks any consideration of color matching with an output image from a printer or the like.
Further, consideration can be given to external light at the point of measuring the monitor device characteristic by using data measured by a non-contact chromometer to generate a profile. However, not only is the measurement difficult, but the versatility of the profile deteriorates because the viewing condition on the monitor is limited.